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CHAPTER 2
Communication in bacteria
Stephen P. Diggle, Stuart A. West, Andy Gardner,
and Ashleigh S. Griffin
2.1 Introduction: communication in a
unicellular world
In 1905, the pioneering plant pathologist E. F. Smith
suggested that ‘a multiple of bacteria are stronger
than a few and thus by union are able to overcome
obstacles too great for the few’ (Smith 1905). This
was for the time a remarkable statement, because
until recently it was considered by most microbiologists that bacterial cells were unicellular organisms that existed in isolation from each other. It is
now well established that bacteria are highly interactive and possess an extraordinary repertoire for
intercellular communication and social behaviours
such as group migration, conjugal plasmid transfer
(sexual transfer of genetic material between cells),
antibiotic resistance, biofilm maturation (development of ‘slime cities’), and virulence which,
although not a social trait, can be a consequence of
social behaviour (Williams et al. 2007).
Indeed, some workers have suggested that these
behaviours are similar to those observed in social
insects, vertebrates, and humans. For example,
Myxococcus xanthus cells exhibit socially dependent swarming across surfaces (Velicer and Yu 2003)
which allows the population to seek out bacterial
prey in a manner reminiscent of hunting packs
of wolves (Dworkin 1996; Crespi 2001). In a similar fashion, biofilms (a collection of bacterial cells
enclosed in a polysaccharide matrix) have been
likened to ant nests and beehives (Crespi 2001;
Diggle et al. 2007b). Furthermore, bacteria such as
Pseudomonas aeruginosa can modulate the immune
response, reminiscent of helminth parasites, and
antibiotic resistance due to the production of
extracellular enzymes (e.g. β-lactamase) could
be considered to be a group defence mechanism
(Diggle et al. 2007b).
Perhaps the paradigm for bacterial cooperation
and communication can be seen in the diverse
quorum sensing (QS) systems found in both Gramnegative and Gram-positive bacteria (Diggle et al.
2007a; Williams et al. 2007). QS describes the phenomenon whereby the accumulation of ‘signalling’
molecules in the surrounding environment enables
a single cell to sense the number of bacteria (cell
density), and therefore the population as a whole
can make a coordinated response. The signal produced regulates its own production (autoinduction)
and so this leads to a positive-feedback response
and greatly increased signal production. At critical
cell densities, the binding of a regulator protein to
the signal leads to the switch on of genes controlled
by QS and a coordinated population response.
It is important to note that many studies on QS
in bacteria have been performed under laboratory
conditions, and it needs to be determined whether
QS is an artefact of laboratory growth (Redfield
2002; Hense et al. 2007). It is possible that this may
be the case for certain organisms, but it has been
shown, for example, that P. aeruginosa makes QS
signal molecules in the lungs of cystic fibrosis
patients (Collier et al. 2002; Middleton et al. 2002).
Despite this, it is still not known whether QS is
important in the development and establishment
of chronic infections in this population. Therefore,
the idea that QS is for the ‘common good’ of the
bacterial population has yet to be significantly
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S O CI O B I O LO GY O F CO M M U N I C AT I O N
Table 2.1 Bacterial cooperative behaviours known to be regulated by QS systems
QS-controlled behaviour
Bacterial species
Biofilms
Aeromonas hydrophila, Burkholderia cenocepacia, Pseudomonas aeruginosa, Pseudomonas
putida, Serratia liquefaciens
Aeromonas hydrophila, Aeromonas salmonicida, Burkholderia pseudomallei, Pseudomonas
aureofaciens, Serratia liquefaciens
Agrobacterium tumefaciens, Rhizobium leguminosarum
Burkholderia cenocepacia, Erwinia carotovora, Chromobacterium violaceum, Pseudomonas
aeruginosa, Serratia spp. ATCC 39006, Serratia proteamaculans
Burkholderia cenocepacia, Pseudomonas aeruginosa, Serratia liquefaciens, Yersina
enterocolitica, Yersinia pseudotuberculosis
Burkholderia cenocepacia
Agrobacterium vitiae, Burkholderia cenocepacia, Burkholderia pseudomallei, Burkholderia
mallei, Erwinia carotovora, Pseudomonas syringae, Pseudomonas aeruginosa
Chromobacterium violaceum, Pseudomonas aureofaciens, Pseudomonas chlororaphis,
Serratia spp. ATCC 39006, Serratia marcescens
Erwinia carotovora, Serratia spp. ATCC 39006
Pantoea stewartii, Pseudomonas syringae
Rhodobacter sphaeroides, Yersinia pseudotuberculosis
Yersinia enterocolitica, Pseudomonas syringae
Rhizobium leguminosarum, Sinorhizobium meliloti
Pseudomonas aeruginosa, Serratia liquefaciens, Serratia marcescens
Serratia marcescens
Vibrio fischeri
Exoproteases
Plasmid conjugation
Exoenzymes
Swarming motility
Siderophore production
Virulence
Pigment production
Antibiotics
Exopolysaccharides
Aggregation
Swimming motility
Root nodulation/symbiosis
Biosurfactant production
Sliding motility
Bioluminescence
proven. That aside, many of the behaviours regulated by QS appear to be cooperative and could
be described as public goods, for example exoenzymes, biosurfactants, antibiotics, and exopolysaccharides (Table 2.1).
The importance of QS to a bacterium can be
seen when studying the opportunistic pathogen
P. aeruginosa. In this organism, a hierarchical QS
system has been estimated to regulate at least 6%
of the genome (Hentzer et al. 2003; Schuster et al.
2003; Wagner et al. 2003) which is a possible reason
why P. aeruginosa is so highly adaptable and able
to inhabit a wide range of diverse environmental
niches.
It is often assumed in the microbiology literature that QS behaviour is cooperative and is for
the good of the population as whole (Shapiro 1998;
Henke and Bassler 2004) and little attention has
been given to the evolutionary implications of QS.
Understanding cooperative behaviour is one of the
greatest challenges faced by evolutionary biologists, and the dictum of the survival of the fittest
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makes it unclear why one organism should behave
for the good of another (Hamilton 1964). This chapter will review QS in bacteria and integrate this
with the literature on animal signalling. We will
discuss the nature of QS signals and signalling
between single species and mixed species (bacterial cross-talk) and whether QS is truly cooperative. We will also explore whether QS in bacteria
can be used to answer fundamental questions,
such as how social behaviours can be maintained
in natural populations.
2.2 When is a signal not a signal?
As will be described later, many diverse compounds
have been identified as bacterial cell-to-cell QS signal molecules. Furthermore, interactions between
different species of bacteria, and even between
prokaryotes and eukaryotes, have also been widely
described. There are several characteristics that a
typical QS signal should display: (1) the production
of the QS signal takes place during specific stages
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of growth or in response to particular environmental changes; (2) the QS signal accumulates in
the extracellular environment and is recognized
by a specific bacterial receptor; (3) the accumulation of a critical threshold concentration of the
QS signal generates a concerted response; and (4)
the cellular response extends beyond the physiological changes required to metabolize or detoxify the molecule (Winzer et al. 2002). Even taking
these factors into consideration, it is also important
to define what a signal is using terminology that
is accepted amongst evolutionary biologists when
discussing signalling between higher organisms
(Keller and Surette 2006; Diggle et al. 2007b) (see
also Chapter 1).
In a seemingly simple scenario, when we see
cell A produce a substance X that elicits a response
in cell B it is tempting to conclude that the substance produced is a signal, i.e. cell A is trying to
tell cell B something. The word ‘signal’ is widely
used to define such substances in the context of
QS, or communication between bacterial cells.
However, broad use of this term can be misleading
and obscure the details of the interaction between
cells that it attempts to describe. This has been well
illustrated by research on communication and signalling in animals, where considerable confusion
has arisen through different researchers using the
same term to mean different things, or different
terms to mean the same things (Maynard Smith
and Harper 2003).
Confusion over terminology can be avoided if
the different kinds of interactions that we observe
when cell A elicits a response in cell B are differentiated, depending upon their consequences for cell
A and cell B (Table 2.2) (Maynard Smith and Harper
Table 2.2 Different types of communication identified by their
fitness consequences on the sender and receiver
Signal
Cue
Coercion
Evolved because of
effect on sender (Cell A)
Benefits receiver
to respond (Cell B)
+
–
+
+
+
–
Beneficial (+), Costly (−).
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2003). Specifically, a signal is defined as ‘characters
that evolve in a signaller in order to provide information to a receiver, aiming to change the behaviour of the receiver to the benefit of the signaller’
(see Chapter 1). This definition distinguishes a signal from a cue, where the production of substance
X by cell A has not evolved because of its effect on
cell B. For example, substance X may be a waste
product produced by cell A that is detected by cell
B. To demonstrate that substance X is a signal and
not a cue it is necessary to show that it evolved
because of the response it elicits. If the production
of substance X by cell A forces a costly response
from cell B we differentiate this from signalling
and term it coercion or chemical manipulation.
Do these semantic points really matter? The
answer is yes, for two reasons. Firstly, it is important
for general understanding if there is a consensus on
the use of terms. This is a lesson hard-learned by
biologists working on signalling in higher organisms (Maynard Smith and Harper 2003), as well
as more generally in the field of social evolution
(West et al. 2007b). Secondly, and more importantly,
we can make very different predictions about the
behaviour of bacterial cells depending on whether
they are communicating by a signal, a cue, or coercion (Table 2.2). For example, if a molecule is a signal, then we can say several things:
1. It is beneficial to cell B to respond.
2. The response of cell B benefits cell A.
3. It might be possible for a signaller to cheat in
the amount of signal that it produces either to: (a)
free-ride on the back of other signallers (avoiding
the cost of producing substance X, i.e. signal negative), (b) manipulate responders (signal can become
coercive), or (c) not respond to the presence of signal and therefore not produce signal-controlled
public goods (signal blind).
4. There must be some mechanism that provides a
shared interest to cells A and B, otherwise cheats
would invade and make the signalling unstable—
later we discuss how kin selection provides a solution to this problem.
5. A signalling system is likely to be more complex
than a system involving a cue, to remain stable in
the face of evolution for individuals to make less
substance X or for individuals to respond less.
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2.3 The discovery of cell-to-cell
communication in bacteria
Whilst the term ‘quorum sensing’ has only been in
use since 1994 (Fuqua et al. 1994), cell-to-cell communication in bacteria has an experimental history
that dates back to the early 1960s. Early work on
fruiting body formation in M. xanthus (Mcvittie
et al. 1962) and on streptomycin production in
Streptomyces griseus (Khokolov et al. 1967) challenged the common view that bacteria behaved as
isolated single cells.
One of the earliest reports of a classical cell
density-dependent phenotype was by Nealson
et al. (1970) who showed that the addition of spent
culture supernatants of the marine luminescent
bacterium Vibrio fischeri (formally Photobacterium
fischeri) to low-density cultures of the same organism induced the production of bioluminescence
due to the presence of a substance they termed an
autoinducer (Nealson et al. 1970). When in a confined area such as a flask, or in symbiosis in a light
organ found in certain species of squid, the autoinducer molecules accumulate to a critical concentration (usually at high bacterial cell densities) which,
in turn, induces expression of the genes responsible for bioluminescence.
The autoinducer responsible for the regulation
of bioluminescence was later identified as N-(3oxohexanoyl) homoserine lactone (3-oxo-C6-HSL)
(Eberhard et al. 1981). The structural and regulatory genes necessary for bioluminescence and
3-oxo-C6-HSL production were identified and
luxR
Amplification
loop
+
luxICDABE
I
R
AHL signal
R
Bioluminescence
Figure 2.1 The LuxR /AHL-driven quorum sensing module of
V. fischeri. LuxR is the AHL receptor and LuxI is the AHL signal
synthase. Many bacteria possess multiple LuxR /LuxI /AHL modules
which work in a similar manner.
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termed the lux regulon (Engebrecht et al. 1983).
This regulon is organized into two divergently
transcribed operons (operons are units of coordinated gene activity which regulate protein synthesis in prokaryotes). The leftward operon comprises
the luxR gene which encodes the transcriptional
regulator protein LuxR. The rightward operon
consists of six genes arranged as luxICDABE. The
luxI gene encodes an autoinducer synthase responsible for the synthesis of 3-oxo-C6-HSL. The luxCDABE genes are involved in generating the products
required for the luciferase reaction and the induction of bioluminescence. The genetic regulation
of bioluminescence in V. fischeri is illustrated in
Fig. 2.1. This elegant mechanism of gene regulation was thought to be a phenomenon restricted to
bioluminescence production in a few marine Vibrio
species; however, it is now known that this type of
system is widespread in Gram-negative bacteria.
In the early 1990s it was discovered that the
production of the β-lactam antibiotic, 1-carbapen2-em-3-carboxylic acid (carbapenem) by the terrestrial plant pathogen Erwinia carotovora was also
regulated by 3-oxo-C6-HSL (Bainton et al. 1992a,b).
This finding led to the intriguing possibility that
many bacteria may use N-acylhomoserine lactones
(AHLs) in order to regulate specific phenotypes.
This was confirmed when Bainton et al. (1992a)
used plasmid-based AHL-biosensors to detect
AHL molecules from spent culture supernatants
from P. aeruginosa, Serratia marcescens, Erwinia herbicola, Citrobacter freundii, Enterobacter agglomerans,
and Proteus mirabilis (Bainton et al. 1992a). Since
this work, many other Gram-negative bacteria have
been shown to produce different types of AHL molecules and all have homologues of LuxI and LuxR
proteins of V. fischeri (Table 2.3). AHL-mediated QS
is responsible for the regulation of a wide variety
of different phenotypes in these organisms.
Although the distribution of Gram-negative
bacteria that produce AHLs is widespread, there
are some Gram-negative species that have failed
to exhibit any activity in any of the AHL biosensor assays available, for example Escherichia coli
and Salmonella species. However, this does not
mean that they are incapable of producing and
sensing a signal, and Gram-negative bacteria
often utilize alternative QS signal molecules. The
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Table 2.3 LuxR/AHL-dependent QS systems in Gram-negative bacteria
Organism
Major AHL(s)
LuxR
LuxI
Phenotypes
Aeromonas hydrophila
Aeromonas salmonicida
Agrobacterium tumefaciens
Agrobacterium vitiae
C4-HSL, C6-HSL
C4-HSL, C6-HSL
3-oxo-C8-HSL
C14:1-HSL,
3-oxo-C16:1-HSL
C6-HSL, C8-HSL
AhyR
AsaR
TraR
AvsR
AhyI
AsaI
TraI
AvsI
Biofilms, exoproteases
Exoprotease
Plasmid conjugation
Virulence
CepR, CciR
CepI, CciI
PmlIR1, BpmR2,
BpmR3
PmlI1, PmlI2,
PmlI3
Exoenzymes, biofilm formation, swarming
motility, siderophore, virulence
Virulence, exoprotease
BmaI1, BmaI3
Virulence
CviI
CarI (ExpI)
Exoenzymes, cyanide, pigment
Carbapenem, exoenzymes, virulence
Exopolysaccharide
Exoenzymes, exotoxins, protein secretion,
biofilms, swarming motility, secondary
metabolites, 4-quinolone signalling,
virulence
Phenazines, protease, colony morphology,
aggregation, root colonization
Phenazine-1-carboxamide
Biofilm formation
Burkholderia cenocepacia
Burkholderia pseudomallei
Burkholderia mallei
C8-HSL, C10-HSL,
3-hydroxy-C8-HSL,
3-hydroxy-C10-HSL,
3-hydroxy-C14-HSL
C8-HSL, C10-HSL
Chromobacterium violaceum
Erwinia carotovora subsp.
carotovora
Pantoea (Erwinia) stewartii
Pseudomonas aeruginosa
C6-HSL
3-oxo-C6-HSL
BmaR1, BmaR3,
BmaR4, BmaR5
CviR
ExpR, CarR
3-oxo-C6-HSL
C4-HSL; C6-HSL,
3-oxo-C12-HSL
EsaR
LasR, RhlR, QscR,
VqsR
EsaI
LasI, RhlI
Pseudomonas aureofaciens
C6-HSL
PhzR, CsaR
PhzI, CsaI
Pseudomonas chlororaphis
Pseudomonas putida
C6-HSL
3-oxo-C10-HSL,
3-oxo-C12-HSL
3-oxo-C6-HSL
PhzR
PpuR
PhzI
PpuI
AhlR
AhlI
CinR, RhiR, RaiR,
TraR, BisR, TriR
CinI, RhiI, RaiI
CerR
SmaR
SwrR
CerI
SmaI
SwrI
SpnR
SpnI
SprR
SinR, ExpR, TraR
SprI
SinI
LuxR
YenR, YenR2
LuxI
YenI
Bioluminescence
Swimming and swarming motility
YpsR, YtbR
YpsI, YtbI
Motility, Aggregation
Pseudomonas syringae
Rhizobium leguminosarum bv.
viciae
Rhodobacter sphaeroides
Serratia spp. ATCC 39006
Serratia liquefaciens MG1
Serratia marcescens SS-1
Serratia proteamaculans B5a
Sinorhizobium meliloti
Vibrio fischeri
Yersinia enterocolitica
Yersinia pseudotuberculosis
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C14:1-HSL, C6-HSL,
C7-HSL, C8-HSL,
3-oxo-C8-HSL,
3-hydroxy-C8-HSL
7-cis-C14-HSL
C4-HSL, C6-HSL
C4-HSL, C6-HSL
C6-HSL, 3-oxo-C6-HSL,
C7-HSL, C8-HSL
3-oxo-C6-HSL
C8-HSL, C12-HSL,
3-oxo-C14-HSL,
3-oxo-C16:1-HSL,
C16:1-HSL, C18-HSL
3-oxo-C6-HSL
C6-HSL, 3-oxo-C6-HSL,
3-oxo-C10-HSL,
3-oxo-C12-HSL,
3-oxo-C14-HSL
C6-HSL, 3-oxo-C6-HSL,
C8-HSL
Exopolysaccharide, swimming motility,
virulence
Root nodulation/symbiosis, plasmid
transfer, growth inhibition; stationary
phase adaptation
Aggregation
Antibiotic, pigment, exoenzymes
Swarming motility, exoprotease, biofilm
development, biosurfactant
Sliding motility, biosurfactant, pigment,
nuclease, transposition frequency
Exoenzymes
Nodulation efficiency, symbiosis,
exopolysaccharide
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cabbage pathogen Xanthomonas campestris employs
a low-molecular-weight diffusible factor unrelated to AHLs to regulate expression of virulence
determinants such as extracellular enzymes and
exopolysaccharide (Barber et al. 1997). Furthermore,
another plant pathogen, Ralstonia solanacearum,
uses a 3-hydroxypalmitic acid methyl ester as
a volatile signal molecule (Clough et al. 1997).
Myxococcus xanthus also produces non-AHL signals. This Gram-negative bacterium is capable of
forming complex multicellular structures that play
a role in starvation survival. In order to coordinate
this, M. xanthus produces two different signals, the
A-signal and the C-signal. The A-signal, produced
under nutrient limitation and at high cell densities,
is the first signal that triggers multicellular behaviour. Analysis has revealed that the A-signal is a
mixture of amino acids and small peptides (Kuspa
et al. 1992). Following the formation of a layer of
cells triggered by the A-signal, the production of
(a)
O
R
(e)
O
N
H
3-oxo-AHL
OH
(b)
HO
O
O
O
O
Al-2
O
OH
O
N
H
O
(c)
AHL
N
H
CH3
HO
3-hydroxy-AHL
R
OH
O
O
(f)
N
H
B
HO
O
R
the C-signal gives rise to the next stages in the
development process, cell aggregation and sporulation.
The molecules identified and the processes controlled in M. xanthus are very different from those
associated with AHLs and there have now been
multiple signalling systems described, using different chemical signals, in the same organism. For
example, P. aeruginosa has been shown to produce
two AHL-distinct classes of molecules (2-alkyl-4quinolones and cyclic dipeptides) with signalling
activity in addition to AHLs (Holden et al. 1999;
Pesci et al. 1999; Diggle et al. 2006a). This suggests
that the signal may be tailored to particular physiological or environmental conditions depending
upon its physical properties. Some examples of
bacterial QS signals can be seen in Fig. 2.2.
Signalling is not restricted to Gram-negative bacteria: a number of Gram-positive bacteria have been
shown to employ small, modified oligopeptides
O
(g)
PQS
O
COOH
(d)
DSF
(h)
HO
O
OH
O
O
A-Factor
O
H3CO
PAME
Figure 2.2 Structures of quorum sensing signal molecules found in bacteria. (a) 3-oxo-AHL; (b) 3-hydroxy-AHL; (c) N-acyl homoserine
lactone (R ranges from C1-C15); (d) A-factor (2-isocapryloyl-3-hydroxy-methyl-γ-butyrolactone; (e) AI-2 (autoinducer-2); (f) The Pseudomonas
quinolone signal (PQS, 2-heptyl-3-hydroxy-4(1H)-quinolone); (g) DSF (diffusible factor, methyl dodecenoic acid); (h) PAME (hydroxyl-palmitic
acid methyl ester).
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as extracellular signalling molecules. These peptides activate gene expression by interacting with
two-component histidine protein kinase signal
transduction systems (Kleerebezem et al. 1997). For
example, in Staphylococcus aureus the expression of
a number of cell density-dependent virulence factors is regulated by the global regulatory locus agr
(accessory gene regulator) (Williams et al. 2007).
2.4 Evolutionary problems of signalling
and cooperation
2.4.1 The problems of communication and
cooperation
Two problems that have received much attention
in the field of evolutionary biology are cooperation and communication (Hamilton 1964; Maynard
Smith and Harper 2003), and these two issues
come together in QS (Brown and Johnstone 2001;
Redfield 2002; Keller and Surette 2006; Diggle et al.
2007b). In this section we consider the conditions
under which QS to coordinate cooperation can be
evolutionarily stable. We base our review of the relevant theory on Diggle et al. (2007b).
The problem of cooperation is why should an
individual carry out a cooperative behaviour that
is costly to perform, but benefits other individuals
or the local group (Hamilton 1964). Such cooperation is vulnerable to invasion by cheaters who do
not cooperate, but gain the benefit from the cooperation of others. This problem is well known in
the fields of economics and human morality, where
it is termed the tragedy of the commons (Hardin
1968). The tragedy is that, as a group, individuals
would do better with cooperation, but this is not
stable because each individual gains by selfishly
pursuing its own short-term interests.
We have recently reviewed this problem in a
microbial context elsewhere (West et al. 2006, 2007a).
An obvious case in which it arises is when cells
produce extracellular products for nutrient acquisition (Dinges et al. 2000; Greig and Travisano 2004;
Griffin et al. 2004), antibiotics (Riley and Wertz
2002), immune modulation molecules (Brown 1999;
Tateda et al. 2003; Hooi et al. 2004), antibiotic degradation compounds (e.g. β-lactamases) (Ciofu et al.
2000), and bio-surfactants (e.g. rhamnolipids) for
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motility (Velicer and Yu 2003; Daniels et al. 2004).
These products are costly to an individual to produce, but provide a benefit to the individuals in the
local group or population. Economic and evolutionary theory refers to such things as public goods
(Dionisio and Gordo 2006). Many bacterial products termed ‘virulence factors’ are likely to be public goods—their coordinated production leading to
damage to the host. The problem in these cases is
that cheaters who do not pay the cost of producing
such goods can still gain the benefit from neighbouring cooperators who do (for an experimental
demonstration see Griffin et al. (2004) and Diggle
et al. (2007c). This makes the cooperative production of public goods unstable, unless a mechanism
such as kin selection operates (see below) (West
and Buckling 2003).
The problem of communication is how can
communication be reliable (Maynard Smith and
Harper 2003)? Why do individuals convey honest information about themselves, to the benefit of
other individuals? Why would they not give a false
signal to their selfish advantage? If communication
isn’t reliable, then why should the receiver listen to
it? The problem is reviewed for communication in
general by Maynard Smith and Harper (2003) and
within the specific context of bacteria by Keller and
Surette (2006) (see also Chapter 1).
2.4.2 The problem of quorum sensing
Quorum sensing is generally assumed to coordinate
cooperative behaviours in bacteria. Specifically, QS
appears to provide a means for individual bacteria
to assess local cell density and to engage in cooperation once a threshold density has been reached.
Many cooperative ventures will not be worthwhile
until a sufficient number of cells are present, so one
would expect facultative cooperation based on the
presence of cues such as QS molecules that act as
a proxy for cell density. The idea is that signalling
molecules are released, and that this rate of release
is further increased by signalling molecules. This
leads to positive feedback at high cell densities, and
a dramatic increase in cooperative effort (Diggle
et al. 2007a; Williams et al. 2007). (See Chapter11
for a related discussion on collective behaviours in
other taxa.)
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However, this communication may potentially be
invaded by cheats that exploit this system (Brown
and Johnstone 2001; Redfield 2002; Keller and
Surette 2006). One possibility is a cheat that does
not produce QS molecules (signal negative), and
so benefits from monitoring the local cell density
without investing effort into the dissemination of
this information. An alternative possibility would
be for a cheat to neither make the costly signal nor
to respond to it (signal blind). A further possibility
is for a signal blind cheat to make a signal but not
respond. The crucial point here is that both signalling and responding to a signal with the production of public goods are costly. Consequently, there
must be benefits that outweigh these—otherwise
the system could be invaded by cheats that did not
signal or cooperate.
As has previously been discussed, there are
many species of bacteria that use QS to regulate
the production of public goods and are therefore
exploitable by cheats. It is important to note that
many P. aeruginosa clinical isolates are ‘signal blind’
(i.e. they may or may not make minimal amounts
QS-controlled
public goods
e.g. Proteases,
virulence factors
of signal but, importantly, do not respond to a
signal) (Denervaud et al. 2004; Smith et al. 2006),
and so it is desirable to understand the costs and
benefits of QS from an empirical perspective. A
fundamental first step is to determine the fitness
consequences of producing and responding to a
signal. Calculating the number of ATP molecules
required to make signal, Keller and Surette (2006)
suggested that the cost of production of QS molecules varies from low to high depending on the
type of signal molecule produced (Keller and
Surette 2006).
Whilst there is undoubtedly a cost in making
a signal, it is likely that the cost of responding is
more metabolically expensive, especially when
you consider that 6% of the P. aeruginosa genome
changes in response to the addition of QS molecules. Given high costs, QS signalling or response
could be potentially exploitable by QS cheats (Keller
and Surette 2006; Diggle et al. 2007b). In theory, QS
cheats could take the form of either: (1) a ‘signal
negative’ strain which does not make the molecule
but can respond to it, or (2) a ‘signal blind’ strain
lasR
lasI
Response protein LasR
LasI Signal synthase
Signal 1 (30-C12-HSL)
rh/R
QS-controlled
public goods
e.g. Proteases,
virulence factors
Rh/R
rh/I
Rh/I
Signal 2 (C4-HSL)
Figure 2.3 The hierarchical quorum sensing (QS) system of P. aeruginosa. The QS cascade is induced at high population cell densities when
within the cell, the LasR response protein binds to a critical concentration of 3O-C12-HSL signal which has been produced by neighboring
cells and taken up from the surrounding environment. This results in activation of the las QS system and the production of a number of
QS-regulated public goods such as the proteases. Activation of the las system is also important in the induction of the rhl QS system which is
also required for the production of proteases and a number of other rhl-controlled public goods.
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which may (or may not) make signal but, more
importantly, does not respond to it.
Recently we have been addressing empirically
(using P. aeruginosa) whether QS is costly and subject to cheating behaviour (Diggle et al. 2007c). In
P. aeruginosa, QS is controlled by two pathways
(homologous to the V. fischeri luxIR system) which
regulate the production of AHL signalling molecules (Fig. 2.3). These two systems are termed
las and rhl, and use different AHL signal molecules, synthesized via LasI [N-(3-oxododecanoyl)homoserine lactone (3O-C12-HSL)], and RhlI
[N-butanoylhomoserine lactone (C4-HSL)], respectively (Latifi et al. 1995, 1996; Winson et al. 1995).
Importantly, in P. aeruginosa QS regulates many
potential social traits such as virulence, biofilm
formation, and swarming motility. To examine the
consequences of QS for social fitness, we focused
on the las QS pathway because this system is top
of the QS hierarchy (Fig. 2.3) (Latifi et al. 1996; Pesci
et al. 1997), and a mutation in the las system results
in the general abolition of QS.
We constructed both signal negative (lasI-) and
signal blind (lasR-) mutants. Importantly, in the
laboratory we can experimentally alter the level
of signal perceived by either the wild type or the
signal negative mutant by adding synthetic signal,
which is chemically identical to that produced by
P. aeruginosa, to cultures (Chhabra et al. 2003). We
first examined the fitness consequences of QS in a
situation where cooperation is favoured. A group
of exoproducts whose production is controlled by
QS in P. aeruginosa are the proteases. We examined
the growth of the wild type and the signal negative
and signal blind mutants in a medium where the
ability to make proteases is required for growth.
We found that: (1) both the signal negative and signal blind mutants grew very poorly in this medium
when compared with the parental wild-type
strain; (b) addition of synthetic signal to the signal negative strain significantly improved growth,
as would be expected, because this will stimulate
the production of proteases; (c) addition of signal
to the signal blind strain resulted in no improvement in growth, as would be expected because
the cells do not respond to the signal (Diggle et al.
2007c). This shows that QS can provide a benefit at
the population level, by increasing the production
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of cooperative exoproducts, that can aid growth in
certain environmental conditions.
We then determined whether the production of the QS signal molecules and cooperative
QS-dependent exoproducts (public goods) is costly.
We did this by comparing the growth rate of the
mutants and the wild type in nutrient-rich Luria–
Bertani (LB) broth, where the exoproducts produced by QS are not needed for growth. In these
conditions, the QS mutants were able to grow to a
higher density than the wild type. Addition of synthetic signal molecule to the signal negative mutant
resulted in growth profiles similar to those seen for
the wild type, suggesting that the response to QS
signal molecules is costly as similar results were
not seen when signal was added to the signal blind
strain. These results suggests that upon entry to the
stationary phase, QS signalling and the production
of QS-dependent public goods place a heavy metabolic load on the cell (Diggle et al. 2007c).
Thus, it can be shown experimentally that QS is
a social trait susceptible to exploitation and invasion by cheats. Given this, how is QS maintained in
natural populations? The most likely explanation
is kin selection, with cooperation being favoured
because it is between close relatives.
2.4.3 A kin selection model of
quorum sensing
Kin selection theory provides an explanation for
cooperation or communication between relatives
(Hamilton 1964). By helping a close relative reproduce, an individual is still passing on its own genes
to the next generation, albeit indirectly. This theory
is formalized by Hamilton’s rule (Hamilton 1964),
which states that altruistic cooperation is favoured
when rb − c > 0; where c is the fitness cost to the
altruist, b is the fitness cost to the beneficiary, and r
is their genetic relatedness. This predicts that individuals should be more likely to cooperate when
social partners are more closely related (higher r).
For example, high levels of production of public
goods are predicted when relatedness is higher
among interacting bacteria (West and Buckling
2003). Relatedness can often be extremely high
in bacteria because limited dispersal and clonal
reproduction can lead to the individuals interacting
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S O CI O B I O LO GY O F CO M M U N I C AT I O N
over a small area being predominantly clone-mates
(West et al. 2006).
Brown and Johnstone (2001) developed a kin
selection model of QS. They assumed:
1. Signalling is costly to the individual. The fitness
of an individual cell is negatively correlated to the
amount of signalling by that individual.
2. The production of public goods, in response to
QS, is costly to the individual. The fitness of an individual cell is negatively correlated to the amount of
public goods produced by that individual.
3. The production of public goods provides a benefit to the local group of interacting cells (the group).
The fitness of an individual cell is positively correlated to the average amount of public goods produced by the local individuals.
4. The benefit of producing public goods is greater
at higher population densities. The fitness benefit
to an individual cell of a certain level of local production of public goods is positively correlated
with cell density.
Brown and Johnstone (2001) then made predictions for the evolutionarily stable level of signalling (production of signalling molecule) and public
goods production (cooperation). A behaviour is
described as an evolutionarily stable strategy
(ESS) if it cannot be invaded or beaten by a mutant
performing any other strategy once it has been
adopted by the majority of individuals (Maynard
1. Result 1. The ESS level of signalling and public goods production both increased with greater
population densities. At low densities there is little to be gained from the cooperative production
of public goods.
2. Result 2. The ESS level of production of public
goods increased with higher relatedness between
interacting bacteria (Fig. 2.4a). This is expected
because greater levels of cooperation are favoured
with a higher relatedness. However, appreciable
levels of cooperation can be predicted even when
relatedness is relatively low.
3. Result 3. The ESS level of signalling showed a
domed relationship with relatedness (Fig. 2.4b).
At high relatedness there is a shared interest in
cooperation, and in cheap signalling. At low relatedness, there is no selection for cooperation, and
hence no selection for signalling to coordinate
this. With intermediate relatedness, there can still
be selection to produce public goods, but it is in
the individual’s interest to produce fewer public
goods than the other local cells (because r < 1). This
favours higher levels of signalling in an attempt to
manipulate competitors to cooperate more (which
in turn leads to the signal being increasingly
ignored). This is termed ‘competitive devaluation
of signal strength’ (Brown and Johnstone 2001).
(b)
Signalling
Cooperation
(a)
Smith and Price 1973). In particular, they examined
the consequences of variation in mean population
density and relatedness (r). They found that:
Relatedness
Relatedness
Figure 2.4 Brown and Johnstone’s theoretical model of quorum signalling. (a) Cooperation effort increases with increasing relatedness,
because the inclusive fitness benefits of cooperation are maximal at high relatedness and minimal at low relatedness. (b) Signalling effort
is a dome-shaped function of relatedness, because at low relatedness there is little inclusive fitness benefit to be accrued from organizing
a cooperative venture, and at high relatedness there is little conflict so that a cheap signal is favoured, whereas at intermediate relatedness
cooperation is worthwhile yet there is also scope for conflict so a costly signal is required to initiate competition.
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Experimentally we tested Brown and Johnstone’s
theory that QS can be maintained by kin selection.
Using a QS-positive wild type (QS positive) and a
signal blind cheat, mixed together (1:1) in a medium
where the ability to quorum sense is essential for
survival, we found that QS was favoured at a relatively high relatedness. This is in agreement with
Brown and Johnstone’s prediction that cooperation
would increase with higher relatedness (Fig. 2.4a).
Under conditions of high relatedness, and a number
of rounds of selection, the wild-type cells constituted 100% of the total population. In contrast, in
conditions of low relatedness, the cheats increased
in frequency to approximately 60% after a number
of rounds of selection. Therefore, low relatedness
within a population allows cheats who do not
quorum sense to exploit the individuals who do
(Diggle et al. 2007c).
2.4.4 Other models of quorum sensing
Brown and Johnstone’s (2001) model provides a
clear and elegant application of kin selection theory to QS. However, as they stress, it makes many
simplifications, the relaxing of which may have
important consequences. Furthermore, much more
has been learnt about QS since, and we should
also consider alternative possible explanations
for QS.
Brown and Johnstone’s (2001) model could be
extended to investigate the consequences of several biological complexities. It has been found that
signalling molecules can have multiple functions,
and this would alter the relative cost and benefit
of their production, as well as how this would
vary with the social context. For example, they
can also function as antibiotics (Stein 2005), potentially as public goods such as iron-scavenging
molecules (Kaufmann et al. 2005; Diggle et al.
2007d), and as potent immune modulators (Tateda
et al. 2003; Hooi et al. 2004). Production and secretion of signal molecules may also be linked to the
production of other molecules through excretion
in membrane vesicles (Mashburn and Whiteley
2005). Another possibility is that different types of
signal need to be considered, with different costs
or specificities. It appears that specificity and cost
vary across signals, with cheap-to-produce signals
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being utilized very generally across species, and
more expensive signals being more specific, within
species, possibly even within lineages (Keller and
Surette 2006).
Kin selection is not the only possible explanation
for cooperation (Sachs et al. 2004; see an individuallevel hypothesis by Zahavi in Chapter1) An alternative explanation for cooperation is that it provides
a direct benefit to the individual performing the
behaviour, which outweighs the cost of performing
the behaviour (i.e. it is mutualistic not altruistic).
An example of this would be if the waste product
of one species provided a benefit to individuals of a
second species (by-product benefit), and hence the
second species could be selected to cooperatively
help individuals of the first species, in order to
increase the by-product benefits (Sachs et al. 2004).
It would be extremely interesting to see whether
communication between species can be evolutionarily stable in such cases. There are several other
forms of direct benefit to cooperation that could be
examined from a QS and communication perspective—for example, when cooperation is stabilized
between non-relatives by policing or punishment
of non-cooperators (Frank 2003).
2.5 Defining signalling in bacteria
As discussed earlier, the fact that a compound
produced by cell A elicits a response in cell B
does not necessarily mean that there is true signalling between the cells and may represent cell
B using the molecule as a ‘cue’ or cell A coercing cell B into a certain action. In this section we
discuss examples of QS between single populations and mixed populations of bacteria and suggest whether this can be considered signalling, a
response to a cue, or a coercion (see also Keller
and Surette 2006).
In general, communication in bacteria can be
divided into three main areas:
1. Intraspecies: communication arising or occurring within a single bacterial species.
2. Interspecies: communication arising between
two or more distinct species of bacteria.
3. Interkingdom: communication arising between
a bacterial species and a higher organism.
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2.5.1 Intraspecies communication
In Gram-negative bacteria, the most intensely studied QS systems rely upon the interaction of AHL
signal molecules synthesized by LuxI-type AHL
synthases, with LuxR-type transcriptional regulator proteins (see Section 2.3). A simple example of
this can be seen in the marine bacterium V. fischeri
(Nealson et al. 1970). This organism forms a symbiotic relationship with the squid Euprymna scolopes
where it colonizes the light organ (McFall-Ngai
and Ruby 2000). At low cell densities the bacterial population does not luminesce but at high
densities there is a coordinated switch on of bioluminescence. This production of light has been
shown to be mediated by a diffusible AHL molecule (3O-C6-HSL) synthesized by the LuxI protein. At a critical concentration, 3O-C6-HSL binds
to LuxR and the complex activates expression of
the luxCDABE operon resulting in coordinated
production of bioluminescence. Under laboratory
conditions, it is possible to stimulate early induction of bioluminescence simply by providing the
cells with exogenous 3O-C6-HSL. It is not entirely
clear why V. fischeri cells have a shared interest
that favours signalling and cooperation to produce
light. Possibilities are a high relatedness between
the cells within a light organ, or the avoidance of
punishment from the host squid if light is not produced (analogous to why rhizobia fix nitrogen for
their host plants (West et al. 2002a; Kiers et al. 2003)).
Indeed, it appears to be the case that the squid can
enforce bioluminescence by altering the environment such that lux-deficient strains are defected
in light organ colonization. It was hypothesized
that a diminished level of oxygen consumption by
lux-deficient strains is responsible for the reduced
fitness (Visick et al. 2000).
As many species of Gram-negative bacteria have
been shown to produce AHL signalling molecules,
then similar examples can be seen in other species
(Diggle et al. 2007a; Williams et al. 2007). Some bacteria have been shown to regulate the production of
virulence determinants in a cell density-dependent
manner. For example, Erwinia carotovora subsp.
carotovora coordinately produces both exoenzymes,
which destroy plant tissue, and the antibiotic carbapenem in response to critical concentrations of
DettoreBook.indb 22
3O-C6-HSL (Jones et al. 1993). Similarly the opportunistic pathogen P. aeruginosa regulates an arsenal
of extracellular virulence factors using a complex hierarchical QS cascade involving two major
AHL molecules, namely 3O-C12-HSL and C4-HSL
(Venturi 2006). In such cases it is likely that these
are examples where QS molecules can be classed as
‘signals’ between cells as the production by cell A
has evolved due to its effects on cell B which in turn
has evolved a response to the signal (Maynard Smith
and Harper 2003). We suspect that kin selection is
the mechanism to explain the evolutionary stability of such signalling, as discussed in Section 2.4.
Although the AHL family of QS molecules have
been described in a wide variety of Gram-negative
bacterial species (Lazdunski et al. 2004), crucially
they tend to differ between bacterial species. AHLs
consist of a conserved homoserine lactone ring
connected via an amide bond to an acyl side chain
which can vary in length from 4 to 18 carbons.
In addition, these side chains may or may not be
modified with a 3-hydroxy or a 3-oxo group, potentially providing a large variety of AHL molecules.
Many species of bacteria will only respond to their
cognate molecule(s) providing a certain degree of
specificity, and therefore AHL signalling is generally of an intraspecies nature. Some bacteria, however, are able to ‘exploit’ AHLs produced by another
species, and this will be discussed later.
Whilst it is plausible to view AHLs as signals
between cells of the same species, the situation is
often more complicated as some AHLs have been
shown to have multiple functions. For example
3O-C12-HSL produced by P. aeruginosa has been
reported to have immunomodulatory properties
(Telford et al. 1998; Tateda et al. 2003). It is unlikely
that this involves signalling between the host and
bacteria. More likely, this represents 3O-C12-HSL
‘chemically manipulating’ or ‘coercing’ the host
immune response to the benefit of the bacterial
population.
The world of microbial communication is not
limited to Gram-negative bacteria. Gram-positive
bacteria also produce QS molecules but tend to
utilize post-translationally modified autoinducing
peptides (AIPs). For example, S. aureus uses AIPs to
regulate the production of exotoxins in response to
a critical concentration of peptide (Novick 2003).
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Explaining within-species cooperative signalling requires some kind of mechanism (see also
Chapter11). The production of a costly signal for
the common good makes this type of communication exploitable by cheats who do not contribute to signal production but reap the benefits of
QS-mediated behaviour, for example acquisition of
nutrients provided by QS-dependent exoenzyme
production. In fact, recent work has shown that
many P. aeruginosa clinical isolates are QS defective
and make very few virulence factors when grown
in the laboratory (Denervaud et al. 2004; Schaber
et al. 2004; Lee et al. 2005) suggesting that it may
be beneficial not to signal under certain environmental conditions, or that cheats can invade in
long-term infections (West et al. 2006). As local
populations of cells are likely to be closely related,
then one way that cooperation can be maintained
is via kin selection, which requires a sufficiently
high relatedness between cooperating individuals
(West et al. 2006). Limited dispersal (population
viscosity) would tend to keep relatives together
(Hamilton 1964). In this case, indiscriminate altruism may be favoured because neighbours will tend
to be relatives (Hamilton 1964; Queller 1992; West
et al. 2002b). This type of mechanism is likely to
be of huge importance in microorganisms where
asexual reproduction means that single cells colonize and grow in a local area. In this case, the individuals interacting over a small area will be clonal,
which can be very conducive to the evolution of
cooperation.
2.5.2 Interspecies communication—bacterial
‘cross-talk’
A third class of QS signal molecule has been
described in the marine bacterium Vibrio harveyi.
Bioluminescence in this organism is cooperatively
regulated by AHLs and a molecule termed autoinducer-2 (AI-2) which is a furanosyl borate diester
produced by the enzyme LuxS (Chen et al. 2002).
The identification of the luxS gene required for the
production of AI-2 production (Surette et al. 1999)
sparked an exponential increase in AI-2 signalling research. The reason being that the luxS gene
can be found in a wide variety of bacterial genera
(Winzer et al. 2002, 2003).
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Importantly, representatives of both Gramnegative and Gram-positive bacteria carry this
particular gene, and consequently AI-2 production has been demonstrated in many species of
bacteria. This has led to the hypothesis that AI-2
is employed as a means of interspecific communication or ‘bacterial Esperanto’ (Winans 2002). This
idea is difficult to explain from an evolutionary
point of view, as cooperation between species is
even harder to explain than within species. The
major difference is that kin selection, as discussed
in Section 2.3, will not be important across species.
There are mechanisms by which cooperation can
be favoured between species, such as by-product
benefit (Sachs et al. 2004), or to avoid punishment
(West et al. 2002a; Kiers et al. 2003), but these are
expected to be rarer (West et al. 2006).
It must therefore be questioned whether AI-2 can
be defined as a true signal. For this to be the case
AI-2 must: (1) be diffused from the cell, (2) be taken
up by a neighbouring cell, (3) elicit a response
from that cell because the receiver’s response has
evolved, (4) benefit both producer and receiver.
Clearly points 1 and 2 are met with respect to AI-2
but there are major doubts about points 3 and 4.
Despite AI-2 being produced by many genera,
there is very little evidence linking it with direct
activation of any specific genes. Studies in many
different bacteria have shown that luxS mutants
differ phenotypically from wild-type strains; however, this can often be explained because of a defect
in a metabolic pathway. It is now well known that
LuxS plays an important role in bacterial metabolism, contributing to the recycling of S-adenosyll-methionine (SAM), of which AI-2 is a metabolic
by-product (Winzer et al. 2003). To date only bioluminescence in V. harveyi (Surette et al. 1999), and an
ABC transporter in Salmonella typhimurium (termed
Lsr) (Taga et al. 2001) have been shown to be regulated by AI-2. In these species, we can speculate
that AI-2 may be used as a cooperative signal in
an intraspecies context. Theoretically, these species
could also use AI-2 from other organisms to regulate these respective traits. In this case, however,
it is inaccurate to use the term interspecies signalling as the receiver’s response has not evolved in
parallel with the producing bacterial species. In
this scenario we can say that both V. harveyi and
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S. typhimurium use the metabolic by-product AI-2
as an environmental ‘cue’ to regulate gene expression. Interspecies signalling has also been suggested between avirulent oropharyngeal flora (OF)
(AI-2 +ve) and P. aeruginosa (luxS and AI-2 –ve)
within the cystic fibrosis (CF) lung (Duan et al.
2003). Co-incubation of P. aeruginosa with OF bacteria resulted in an increase in virulence gene
expression which was attributed, at least in part,
to AI-2. The mechanism for this is unknown as
P. aeruginosa does not make AI-2 but we suggest
that this is not an example of interspecies signalling. It is more likely that P. aeruginosa is able to use
AI-2 as a cue, perhaps to assess its surroundings,
or it may be that OF bacteria ‘coerce’ or manipulate
P. aeruginosa into increased virulence which may
provide them with more nutrients.
Interspecies signalling between bacterial species
using AHL molecules has also been suggested.
Pseudomonas aeruginosa and Burkholderia cepacia
often occur together in the lungs of people with
cystic fibrosis, where they are associated with high
morbidity and mortality (Eberl and Tummler 2004;
Govan and Deretic 1996). Burkholderia cepacia has
been shown to up-regulate the production of virulence determinants in response to AHLs produced
by P. aeruginosa, although this does not appear to
happen the other way round. This type of behaviour
has also been termed ‘bacterial cross-talk’ which is
suggestive of a cooperative venture between two or
more species. In this case, it suggests that B. cepacia
uses P. aeruginosa AHLs as a cue to alter its behaviour rather than there being signalling between the
two bacterial species. Pseudomonas aeruginosa pays
the cost of producing AHLs, possibly for withinspecies signalling, but appears to gain no benefit
from B. cepacia in return.
2.5.3 Interkingdom communication across the
prokaryote/eukaryote divide
Several recent reports have demonstrated that bacterial QS molecules (specifically AHLs) can affect
gene expression in eukaryotes as many eukaryotic
hormones structurally resemble AHLs. Generally
this has been termed interkingdom signalling or
global sensing (Shiner et al. 2005). AHL molecules
have been experimentally demonstrated to affect
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a number of animal cell types including murine
and human primary cells (Telford et al. 1998),
breast cancer cells (Li et al. 2004), bone marrow
macrophages (Tateda et al. 2003), and primary porcine arterial smooth muscle cells (Lawrence et al.
1999). In addition, plant behaviour has also been
shown to be modified by AHLs. The zoospores
of the seaweed Enteromorpha have been shown to
settle preferentially on AHL-producing biofilms
of the marine bacterium Vibrio anguillarum (Joint
et al. 2002). Furthermore, higher organisms have
mechanisms that appear to downregulate QS in
microorganisms. For example, the marine red alga
Delisea pulchra produces a halogenated furanone
that disrupts QS in several species of bacteria
including the swarming motility of Serratia liquefaciens (Givskov et al. 1996). This furanone has
also been shown to disrupt P. aeruginosa biofilms
(Hentzer et al. 2002). These AHL ‘mimics’ attract
interest as possible alternatives to antibiotic therapy. Whether these examples demonstrate signalling using small molecules between prokaryotes
and eukaroytes is open to debate. In general, studies performed to date appear to show that either (1)
the signalling bacterium manipulates or coerces its
host into a certain action rather than there being a
truly evolved signalling system between the two
(cf. coercion strategies, Chapters 4 and 10) or (2)
as in the example of the zoospore settlement, the
eukaryote utilizes bacterial AHLs as an environmental cue as a guide to future action.
2.6 Complexities of bacterial
communication
In agreement with behavioural studies on organisms such as birds, mammals, and insects, signalling in bacteria has a number of complexities that
offer problems from an evolutionary perspective.
First, the signal can be degraded (as also occurs for
other modalities such as sound and pheromones).
This degradation can be environmental in nature
or due to the action of certain enzymes. This signal
interference has often been suggested as a possible way of controlling the virulence of pathogenic
bacterial species (i.e. breaking the lines of communication) and thus leading to novel therapies.
AHL signals are rendered biologically inactive in
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alkaline environments (Yates et al. 2002) and therefore, in certain environmental niches, signalling
may be ineffective. In theory, the levels of QS signalling may be greatly influenced by environmental conditions but whether this alters the cost and
benefit of either making a signal, or responding,
has not been explored. AHLs can also be degraded
by enzymes produced by bacteria, a process
known as quorum quenching (Dong and Zhang
2005). Examples include AiiA, an AHL lactonase
produced by a Bacillus spp. (Dong et al. 2001), and
PvdQ, an AHL-acylase produced by P. aeruginosa
(Sio et al. 2006). This raises many interesting questions, which could be empirically tested. What effect
can an AHL-degrading species have on an AHL
producer? For instance, does degradation interfere with key social behaviours such as population
swarming or result in the reduction of a number
of harmful AHL-dependent exoproducts which is
ultimately beneficial to the degrading organism?
Can this behaviour be considered coercive or spiteful, and are there indirect or direct fitness benefits
for the AHL degrader? Is AHL degradation evolutionary stable or is it subject to invasion by cheats
who do not make the degrading enzymes?
Second, the genes required for signal generation
(luxI homologues) and response (luxR homologues)
are not always found on the bacterial chromosome.
A number of these homologues have been identified
on plasmids such as the Agrobacterium Ti plasmid
(Zhang et al. 1993) and Rhizobium symbiotic plasmids (Smith 2001; Wisniewski-Dye and Downie
2002). While this may just represent an easy way to
obtain QS mechanisms, could it also be a mechanism by which signalling is forced onto a cell that
doesn’t contain the QS machinery, coercing it into
cooperative behaviour? An important point here is
the conflicting interests of the bacteria involved,
and the plasmids themselves. Third, QS molecules
are not just signals. A number of other roles have
been assigned to QS molecules which suggests they
can also function as public goods, for example iron
chelators (Diggle et al. 2007d), immunomodulatory
compounds(Pritchard 2006), and biosurfactants
(Daniels et al. 2006). QS compounds can also be
harmful or spiteful, for example the AIP lantibiotics typified by lactococcal nisin and produced by
Lactococcus lactis are potent bacteriocides against
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many Gram-positive organisms (Stein 2005). The
consequences of QS signals having multiple functions needs to be explored theoretically (Brown
and Johnstone 2001; Diggle et al. 2007b).
Another complexity of studying signalling in
bacteria is that most bacterial species are capable
of forming structured multicellular communities
known as biofilms (Kolter and Greenberg 2006).
Biofilms are ubiquitous, being found in such diverse
environments as dental plaques, wounds, rock
surfaces, and at the bottom of rivers. They have a
definite structure, including water channels, which
may involve a number of different ‘specialist’ cells
and they are often enclosed by a exopolysaccharide
matrix which can make them difficult to eradicate.
It is also comparatively harder to empirically study
cells growing in a biofilm compared with planktonic cells. However, biofilms are of particular
interest from an evolutionary perspective, because
the close proximity of individuals in a biofilm can
make cooperation and communication particularly
important.
Many forms of cooperation can be involved in
the establishment and growth of a biofilm, such
as the cooperative production of an extracellular
matrix which surrounds the biofilm, and may be
important in maintaining structure (Davies and
Geesey 1995; Nivens et al. 2001; Friedman and Kolter
2004; Matsukawa and Greenberg 2004; Diggle et al.
2006b). In addition, numerous other public goods
can be important in biofilms, such as rhamnolipid,
a biosurfactant which aids in biofilm detachment
(Boles et al. 2005), and microvesicles which are a
component of the extracellular matrix and can
contain signal molecules and proteases (Schooling
and Beveridge 2006). Quorum sensing may play an
important role in the development and structuring
of biofilms produced by certain bacterial species,
as suggested by the poor biofilm formation of some
QS mutants (Davies et al. 1998), although, perhaps
surprisingly, not a great deal is known generally
about QS and biofilm development which may
stem from the fact that biofilms are difficult to
study experimentally. However, it has been shown
in P. aeruginosa that QS plays a role in biofilm differentiation (Fig. 2.5).
The evolutionary implications of QS in biofilms
are also uncertain. It could be expected that kin
6/11/2008 11:12:11 AM
26
S O CI O B I O LO GY O F CO M M U N I C AT I O N
(a)
Dispersal:
Planktonic cells
Surface
attachment
Cell-cell signalling:
Biofilm maturation
Microcolony
formation
(b)
(c)
Figure 2.5 Lifecycle of a bacterial biofilm. (a) Planktonic cells are released from mature biofilms, and via motility mechanisms they settle
on a new surface. Cells become irreversibly attached and begin to form microcolonies. Mechanisms such as cell-cell signalling systems lead
to the differentiation of mature biofilm structures. Diagram adapted from (Kolter and Losick 1998). (b) Scanning electron microscopy image
of P. aeruginosa attachment to stainless steel coupon. The formation of microcolonies can be observed (MC), image taken from (Diggle et al.
2006b). (c) Scanning confocal microscopy image of mature 5 day old P. aeruginosa biofilms grown in flow cell chambers. Image courtesy of
S. Crusz.
selection is important in biofilms initiated by one
or a small number of clonal lineages. However,
naturally formed biofilms very rarely contain just
one species of bacteria, let alone a single clonal lineage. For example, the colonization of human teeth
and the oral mucosa can involve up to 500 species
of bacteria (Kolenbrander et al. 2002). Nonetheless,
kin selection may still be important in such cases
if social interactions take place on a local scale. For
example, if the benefit of producing the materials
that structure the biofilm, such as exopolysaccharides, or other public goods (perhaps regulated
by QS), are shared primarily with neighbouring
cells, then the clonal growth of bacteria means
that these benefits can still be shared with close
relatives (Xavier and Foster 2007). In this case it
DettoreBook.indb 26
might be useful to think of biofilms as consisting
of a number of clonal lineages (groups of lineages),
with cooperation primarily within lineages but
competition primarily between lineages.
Some workers suggest that QS is not true cellto-cell signalling and that is an artefact of laboratory conditions. Redfield (2002), has argued that
autoinducer molecules are not released to signal
to other cells. Redfield suggests that autoinducer
secretion and response may have a more direct
benefit, by allowing individual cells to determine
how rapidly secreted molecules move away from
the cell. This diffusion sensing (DS) could allow
cells to regulate secretion of costly public goods
to minimize losses owing to extracellular diffusion and mixing. This is an alternative explanation
6/11/2008 11:12:11 AM
CO M M U N I C AT I O N I N B AC T ER I A
for the evolution of QS, and diffusion effects could
also be incorporated into kin selection models. It
is important to consider how QS and DS may have
evolved. In the case of QS, it may be assumed that
it evolved because of group benefits, but when you
consider DS this is likely to have evolved because
of benefits to the individual. It should also be considered that production of these molecules may
have initially evolved for one reason (e.g. DS), but
is now maintained for another (e.g. QS). Hense et al.
(2007) recently introduced a new term: efficiency
sensing (ES) which unifies the QS and DS theories. Efficiency sensing argues that sensing will
have been favoured by both individual and group
benefits as the cells measure a combination of cell
density, mass-transfer properties, and spatial cell
distribution (Hense et al. 2007). The hypotheses
need not be alternatives, as it may be the case that
benefits of DS are crucial for the maintenance of
this trait, yet they are still monitored for QS purposes. It is likely that both functions will be of
importance in understanding when and why these
molecules are produced.
2.7 Conclusions and future
perspectives
Quorum sensing systems are widespread amongst
Gram-negative and Gram-positive bacterial species. However, when one compares the microbiological literature on QS with the animal literature
it is sometimes questionable as to whether QS in
bacteria should always be regarded as signalling.
It is likely that QS within species represents signalling, because the natural history of many microorganisms means that the interactions will be
between close relatives and therefore cooperation
and signalling can be explained by kin selection.
However, QS is often described between species or
even across kingdoms, and in these cases QS signals may be used as chemical cues or as coercion
molecules. It is possible that these interactions may
be due to signalling, but cooperation between species requires special conditions that are only rarely
met. There is therefore increasing uncertainty as
to what form of communication QS represents in
bacteria and the challenges for the future will rely
on experimental studies that examine the costs and
DettoreBook.indb 27
27
benefits of communication to both the sender and
responder. Furthermore, cooperation and communication need to be expanded empirically into biofilms as this is the natural state of growth for many
species of bacteria. It is important to note that in one
particular environment where biofilms are formed
(the cystic fibrosis lung), P. aeruginosa QS signal
blind mutants are often isolated; the reasons for this
are poorly understood. Understanding the interactions between strains found within such environments will provide unique insights into eradicating
problematic organisms such as P. aeruginosa.
Summary
The term quorum sensing (QS) is used to describe
communication between bacterial cells, whereby
a coordinated population response is controlled
by diffusible signal molecules. Quorum sensing
has not only been described between cells of the
same species (intraspecies), but also between bacterial species (interspecies), and between bacteria
and higher organisms (interkingdom). Here we
compare the evolutionary literature on animal signalling and cooperation with the microbiological
literature on QS, and discuss whether bacterial QS
can be considered true signalling. From an evolutionary perspective, intraspecies signalling can be
explained using models such as kin selection, but
explanations become more difficult when communication is described between species. It is likely
that this often involves QS molecules being used as
‘cues’ by other species as a guide to future action
or as coercing molecules whereby one species will
‘coerce’ another into a response.
Acknowledgements
We would like to gratefully acknowledge the Royal
Society as S.P.D., S.A.W., A.G., and A.S.G. are all
supported by Royal Society Fellowships.
References
Bainton, N.J., Bycroft, B.W., Chhabra, S.R. et al. (1992a).
A general role for the Lux autoinducer in bacterialcell signaling—control of antibiotic biosynthesis in
Erwinia. Gene, 116, 87–91.
6/11/2008 11:12:12 AM
28
S O CI O B I O LO GY O F CO M M U N I C AT I O N
Bainton, N.J, Stead, P., Chhabra, S.R. et al. (1992b). N-(3oxohexanoyl)-L-homoserine lactone regulates carbapenem antibiotic production in Erwinia carotovora.
Biochemical Journal, 288, 997–1004.
Barber, C.E, Tang, J.L, Feng, J.X. et al. (1997). A novel regulatory system required for pathogenicity of Xanthomonas
campestris is mediated by a small diffusible signal molecule. Molecular Microbiology, 24, 555–566.
Boles, B.R, Thoendel, M., and Singh, P.K. (2005).
Rhamnolipids mediate detachment of Pseudomonas
aeruginosa from biofilms. Molecular Microbiology, 57,
1210–1223.
Brown, S.P. (1999). Cooperation and conflict in hostmanipulating parasites. Proceedings of the Royal Society
Series B: Biological Sciences, 266, 1899–1904.
Brown, S.P. and Johnstone, R.A. (2001). Cooperation in
the dark: signalling and collective action in quorumsensing bacteria. Proceedings of the Royal Society Series B;
Biological Sciences, 268, 961–965.
Chen, X, Schauder, S, Potier, N. et al. (2002). Structural
identification of a bacterial quorum-sensing signal
containing boron. Nature, 415, 545–549.
Chhabra, S.R, Harty, C., Hooi, D.S.W. et al. (2003). Synthetic
analogues of the bacterial signal (quorum sensing)
molecule N-(3-oxododecanoyl)-L-homoserine lactone
as immune modulators. Journal of Medicinal Chemistry,
46, 97–104.
Ciofu, O., Beveridge, T.J., Kadurugamuwa, J, WaltherRasmussen, J., and Hoiby, N. (2000). Chromosomal
beta-lactamase is packaged into membrane vesicles
and secreted from Pseudomonas aeruginosa. Journal of
Antimicrobial Chemotherapy, 45, 9–13.
Clough, S.J., Flavier, A.B., Schell, M.A., and Denny, T.P.
(1997). Differential expression of virulence genes and
motility in Ralstonia (Pseudomonas) solanacearum during exponential growth. Applied and Environmental
Microbiology, 63, 844–850.
Collier, D.N., Anderson, L., McKnight, S.L. et al. (2002). A
bacterial cell to cell signal in the lungs of cystic fibrosis
patients. FEMS Microbiology Letters, 215, 41–46.
Crespi, B.J. (2001). The evolution of social behavior in
microorganisms. Trends in Ecology and Evolution, 16,
178–183.
Daniels, R, Vanderleyden, J., and Michiels, J. (2004).
Quorum sensing and swarming migration in bacteria.
FEMS Microbiology Reviews, 28, 261–289.
Daniels, R, Reynaert, S, Hoekstra, H. et al. (2006). Quorum
signal molecules as biosurfactants affecting swarming
in Rhizobium etli. Proceedings of the National Academy of
Sciences of the USA, 103, 14965–14970.
Davies, D.G. and Geesey, G.G. (1995). Regulation of
the alginate biosynthesis gene algC in Pseudomonas
DettoreBook.indb 28
aeruginosa during biofilm development in continuous
culture. Applied and Environmental Microbiology, 61,
860–867.
Davies, D.G., Parsek, M.R, Pearson, J.P., Iglewski, B.H.,
Costerton, J.W., and Greenberg, E.P. (1998). The involvement of cell-to-cell signals in the development of a bacterial biofilm. Science, 280, 295–298.
Denervaud, V., TuQuoc, P, Blanc, D. et al. (2004).
Characterization of cell-to-cell signaling-deficient
Pseudomonas aeruginosa strains colonizing intubated
patients. Journal of Clinical Microbiology, 42, 554–562.
Diggle, S.P., Cornelis, P., Williams, P., and Camara, M.
(2006a). 4-Quinolone signalling in Pseudomonas aeruginosa: Old molecules, new perspectives. International
Journal of Medical Microbiology, 296, 83–91.
Diggle, S.P., Stacey, R.E., Dodd, C., Camara, M., Williams,
P., and Winzer, K. (2006b). The galactophilic lectin, LecA,
contributes to biofilm development in Pseudomonas
aeruginosa. Environmental Microbiology, 8, 1095–1104.
Diggle, S.P., Crusz, S.A., and Camara, M. (2007a). Quorum
Sensing. Current Biology, 17, R907–R910.
Diggle, S.P., Gardner, A., West, S.A., and Griffin, A.S.
(2007b). Evolutionary theory of bacterial quorum
sensing: when is a signal not a signal? Philosophical
Transactions of the Royal Society B: Biological Sciences,
362, 1241–1249.
Diggle, S.P., Griffin, A.S., Campbell, G.S., and West, S.A.
(2007c). Cooperation and conflict in quorum sensing
bacterial populations. Nature, 450, 411–414.
Diggle, S.P., Matthijs, S., Wright, V.J. et al. (2007d). The
Pseudomonas aeruginosa 4-quinolone signal molecules
HHQ and PQS play multifunctional roles in quorum
sensing and iron entrapment. Chemistry and Biology, 14,
87–94.
Dinges, M.M., Orwin, P.M., and Schlievert, P.M. (2000).
Exotoxins of Staphylococcus aureus. Clinical Microbiology
Reviews, 13, 16–34.
Dionisio, F. and Gordo, I. (2006). The tragedy of the commons, the public goods dilemma, and the meaning
of rivalry and excludability in evolutionary biology.
Evolutionary Ecology Research, 8, 321–332.
Dong, Y.H. and Zhang, L.H. (2005). Quorum sensing and
quorum-quenching enzymes. Journal of Microbiology,
43, 101–109.
Dong, Y.H., Wang, L.H., Xu, J.L., Zhang, H.B., Zhang, X.F.,
and Zhang, L.H. (2001). Quenching quorum-sensingdependent bacterial infection by an N-acyl homoserine
lactonase. Nature, 411, 813–817.
Duan, K.M., Dammel, C., Stein, J., Rabin, H., and Surette,
M.G. (2003). Modulation of Pseudomonas aeruginosa gene
expression by host microflora through interspecies
communication. Molecular Microbiology, 50, 1477–1491.
6/11/2008 11:12:12 AM
CO M M U N I C AT I O N I N B AC T ER I A
Dworkin, M. (1996). Recent advances in the social
and developmental biology of the myxobacteria.
Microbiological Reviews, 60, 70–102.
Eberhard, A., Burlingame, A.L., Eberhard, C., Kenyon, G.L.,
Nealson, K.H., and Oppenheimer, N.J. (1981). Structural
identification of autoinducer of Photobacterium fischeri
luciferase. Biochemistry, 20, 2444–2449.
Eberl, L. and Tummler, B. (2004). Pseudomonas aeruginosa
and Burkholderia cepacia in cystic fibrosis: genome evolution, interactions and adaptation. International Journal
of Medical Microbiology, 294, 123–131.
Engebrecht, J., Nealson, K., and Silverman, M. (1983).
Bacterial bioluminescence—isolation and genetic analysis of functions from Vibrio fischeri. Cell, 32, 773–781.
Frank, S.A. (2003). Perspective: repression of competition and the evolution of cooperation. Evolution, 57,
693–705.
Friedman, L. and Kolter, R. (2004). Two genetic loci produce distinct carbohydrate-rich structural components
of the Pseudomonas aeruginosa biofilm matrix. Journal of
Bacteriology, 186, 4457–4465.
Fuqua, W.C., Winans, S.C., and Greenberg, E.P. (1994).
Quorum sensing in bacteria—the LuxR-LuxI family
of cell density-responsive transcriptional regulators.
Journal of Bacteriology, 176, 269–275.
Givskov, M., DeNys, R., Manefield, M. et al. (1996).
Eukaryotic interference with homoserine lactonemediated prokaryotic signaling. Journal of Bacteriology,
178, 6618–6622.
Govan, J.R.W. and Deretic, V. (1996). Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa
and Burkholderia cepacia. Microbiological Reviews, 60,
539–574.
Greig, D. and Travisano, M. (2004). The prisoner’s dilemma
and polymorphism in yeast SUC genes. Proceedings
of the Royal Society Series B: Biological Sciences, 271,
S25–S26.
Griffin, A.S, West, S.A., and Buckling, A. (2004).
Cooperation and competition in pathogenic bacteria.
Nature, 430, 1024–1027.
Hamilton, W.D. (1964). Genetical evolution of social
behaviour, I & II. Journal of Theoretical Biology, 7, 1–52.
Hardin, G. (1968). Tragedy of the commons. Science, 162,
1243–1248.
Henke, J.M. and Bassler, B.L. (2004). Bacterial social
engagements. Trends in Cell Biology, 14, 648–656.
Hense, B.A, Kuttler, C, Muller, J, Rothballer, M, Hartmann,
A., and Kreft, J.U. (2007). Does efficiency sensing
unify diffusion and quorum sensing? Nature Reviews
Microbiology, 5, 230–239.
Hentzer, M, Riedel, K, Rasmussen, T.B. et al. (2002).
Inhibition of quorum sensing in Pseudomonas aeruginosa
DettoreBook.indb 29
29
biofilm bacteria by a halogenated furanone compound.
Microbiology, 148, 87–102.
Hentzer, M, Wu, H, Andersen, J.B. et al. (2003). Attenuation
of Pseudomonas aeruginosa virulence by quorum sensing inhibitors. EMBO Journal, 22, 3803–3815.
Holden, M.T.G, Chhabra, S.R, de Nys, R. et al. (1999).
Quorum-sensing cross talk: isolation and chemical
characterization of cyclic dipeptides from Pseudomonas
aeruginosa and other Gram-negative bacteria. Molecular
Microbiology, 33, 1254–1266.
Hooi, D.S.W, Bycroft, B.W, Chhabra, S.R, Williams, P., and
Pritchard, D.I. (2004). Differential immune modulatory
activity of Pseudomonas aeruginosa quorum-sensing signal molecules. Infection and Immunity, 72, 6463–6470.
Joint, I., Tait, K., Callow, M.E. et al. (2002). Cell-to-cell communication across the prokaryote-eukaryote boundary. Science, 298, 1207–1207.
Jones, S., Yu, B., Bainton, N.J. et al. (1993). The Lux autoinducer regulates the production of exoenzyme virulence
determinants in Erwinia carotovora and Pseudomonas
aeruginosa. EMBO Journal, 12, 2477–2482.
Kaufmann, G.F., Sartorio, R, Lee., S.H. et al. (2005).
Revisiting quorum sensing: Discovery of additional
chemical and biological functions for 3-oxo-N-acylhomoserine lactones. Proceedings of the National Academy
of Sciences of the USA, 102, 309–314.
Keller, L. and Surette, M.G. (2006). Communication in
bacteria: an ecological and evolutionary perspective.
Nature Reviews Microbiology, 4, 249–258.
Khokolov, A.S., Tovarova, I.I., Borisova, L.N. et al. (1967).
A-factor assuring the biosynthesis of streptomycin by
a mutant strain of Actinomyces streptomycini. Dokladi Na
Bolgarskata Akademiya Na Naukite, 1777, 232–235.
Kiers, E.T., Rousseau, R.A., West, S.A., and Denison,
R.F. (2003). Host sanctions and the legume-rhizobium
mutualism. Nature, 425, 78–81.
Kleerebezem, M., Quadri, L.E.N., Kuipers, O.P., and
deVos, W.M. (1997). Quorum sensing by peptide pheromones and two-component signal-transduction systems in Gram-positive bacteria. Molecular Microbiology,
24, 895–904.
Kolenbrander, P.E., Andersen, R.N, Blehert, D.S, Egland,
P.G, Foster, J.S., and Palmer, R.J. (2002). Communication
among oral bacteria. Microbiology and Molecular Biology
Reviews, 66, 486–505.
Kolter, R. and Losick, R. (1998). One for all and all for one.
Science, 280, 226–227.
Kolter, R. and Greenberg, E.P. (2006). Microbial sciences—
the superficial life of microbes. Nature, 441, 300–302.
Kuspa, A., Plamann, L., and Kaiser, D. (1992). A-signaling
and the cell-density requirement for Myxococcus xanthus
development. Journal of Bacteriology, 174, 7360–7369.
6/11/2008 11:12:12 AM
30
S O CI O B I O LO GY O F CO M M U N I C AT I O N
Latifi, A., Winson, M.K., Foglino, M. et al. (1995). Multiple
homologs of LuxR and LuxI control expression of
virulence determinants and secondary metabolites
through quorum sensing in Pseudomonas aeruginosa
PAO1. Molecular Microbiology, 17, 333–343.
Latifi, A., Foglino, M., Tanaka, K., Williams, P., and
Lazdunski, A. (1996). A hierarchical quorum-sensing
cascade in Pseudomonas aeruginosa links the transcriptional activators LasR and RhIR (VsmR) to expression
of the stationary-phase sigma factor RpoS. Molecular
Microbiology, 21, 1137–1146.
Lawrence, R.N., Dunn, W.R., Bycroft, B. et al. (1999). The
Pseudomonas aeruginosa quorum-sensing signal molecule, N-(3-oxododecanoyl)-L-homoserine lactone,
inhibits porcine arterial smooth muscle contraction.
British Journal of Pharmacology, 128, 845–848.
Lazdunski, A.M., Ventre, I., and Sturgis, J.N. (2004).
Regulatory circuits and communication in gram-negative bacteria. Nature Reviews Microbiology, 2, 581–592.
Lee, B., Haagensen, J.A.J., Ciofu, O., Andersen, J.B., Hoiby,
N., and Molin, S. (2005). Heterogeneity of biofilms
formed by nonmucoid Pseudomonas aeruginosa isolates
from patients with cystic fibrosis. Journal of Clinical
Microbiology, 43, 5247–5255.
Li, L., Hooi, D, Chhabra, S.R, Pritchard, D. and Shaw, P.E.
(2004). Bacterial N-acylhomoserine lactone-induced
apoptosis in breast carcinoma cells correlated with
down-modulation of STAT3. Oncogene, 23, 4894–4902.
McFall-Ngai, M.J. and Ruby, E.G. (2000). Developmental
biology in marine invertebrate symbioses. Current
Opinion in Microbiology, 3, 603–607.
McVittie, A., Zahler, S.A., and Messik, F. (1962).
Developmental biology of Myxococcus. Journal of
Bacteriology, 84, 546–551.
Mashburn, L.M. and Whiteley, M. (2005). Membrane vesicles traffic signals and facilitate group activities in a
prokaryote. Nature, 437, 422–425.
Matsukawa, M. and Greenberg, E.P. (2004). Putative
exopolysaccharide
synthesis
genes
influence
Pseudomonas aeruginosa biofilm development. Journal of
Bacteriology, 186, 4449–4456.
Maynard Smith, J. and Harper, D. (2003). Animal Signals.
Oxford: Oxford University Press.
Maynard Smith, J. and Price, G.R. (1973). The logic of animal conflict. Nature, 246, 15–18.
Middleton, B., Rodgers, H.C., Camara, M., Knox, A.J.,
Williams, P., and Hardman, A. (2002). Direct detection
of N-acylhomoserine lactones in cystic fibrosis sputum. FEMS Microbiology Letters, 207, 1–7.
Nealson, K.H., Platt, T., and Hastings, J.W. (1970). Cellular
control of synthesis and activity of bacterial luminescent system. Journal of Bacteriology, 104, 313–322.
DettoreBook.indb 30
Nivens, D.E., Ohman, D.E., Williams, J., and Franklin,
M.J. (2001). Role of alginate and its O acetylation in formation of Pseudomonas aeruginosa microcolonies and
biofilms. Journal of Bacteriology, 183, 1047–1057.
Novick, R.P. (2003). Autoinduction and signal transduction in the regulation of staphylococcal virulence.
Molecular Microbiology, 48, 1429–1449.
Pesci, E.C., Pearson, J.P., Seed, P.C., and Iglewski, B.H.
(1997). Regulation of las and rhl quorum sensing in
Pseudomonas aeruginosa. Journal of Bacteriology, 179,
3127–3132.
Pesci, E.C., Milbank, J.B.J., Pearson, J.P. et al. (1999).
Quinolone signaling in the cell-to-cell communication
system of Pseudomonas aeruginosa. Proceedings of the
National Academy of Sciences of the USA, 96, 11229–11234.
Pritchard, D.I. (2006). Immune modulation by
Pseudomonas aeruginosa quorum-sensing signal molecules. International Journal of Medical Microbiology, 296,
111–116.
Queller, D.C. (1992). Does population viscosity promote
kin selection. Trends in Ecology and Evolution, 7, 322–324.
Redfield, R.J. (2002). Is quorum sensing a side effect of
diffusion sensing? Trends in Microbiology, 10, 365–370.
Riley, M.A. and Wertz, J.E. (2002). Bacteriocin diversity:
ecological and evolutionary perspectives. Biochimie, 84,
357–364.
Sachs, J.L., Mueller, U.G., Wilcox, T.P., and Bull, J.J. (2004).
The evolution of cooperation. Quarterly Review of
Biology, 79, 135–160.
Schaber, J.A., Carty, N.L., McDonald, N.A. et al. (2004).
Analysis of quorum sensing-deficient clinical isolates of Pseudomonas aeruginosa. Journal of Medical
Microbiology, 53, 841–853.
Schooling, S.R. and Beveridge, T.J. (2006). Membrane
vesicles: an overlooked component of the matrices of
biofilms. Journal of Bacteriology, 188, 5945–5957.
Schuster, M., Lostroh, C.P., Ogi, T., and Greenberg, E.P.
(2003). Identification, timing, and signal specificity
of Pseudomonas aeruginosa quorum-controlled genes:
a transcriptome analysis. Journal of Bacteriology, 185,
2066–2079.
Shapiro, J.A. (1998). Thinking about bacterial populations as multicellular organisms. Annual Review of
Microbiology, 52, 81–104.
Shiner, E.K., Rumbaugh, K.P., and Williams, S.C. (2005).
Interkingdom signaling: Deciphering the language of
acyl homoserine lactones. FEMS Microbiology Reviews,
29, 935–947.
Sio, C.F., Otten, L.G., Cool, R.H. et al. (2006). Quorum
quenching by an N-acyl-homoserine lactone acylase
from Pseudomonas aeruginosa PAO1. Infection and
Immunity, 74, 1673–1682.
6/11/2008 11:12:12 AM
CO M M U N I C AT I O N I N B AC T ER I A
Smith, E.E., Buckley, D.G., Wu, Z.N. et al. (2006). Genetic
adaptation by Pseudomonas aeruginosa to the airways
of cystic fibrosis patients. Proceedings of the National
Academy of Sciences of the USA, 103, 8487–8492.
Smith, E.F. (1905). Bacteria in relation to plant disease. Carnegie Institution Report. Carnegie Institute of
Washington, Washington, DC.
Smith, J. (2001). The social evolution of bacterial pathogenesis. Proceedings of the Royal Society Series B: Biological
Sciences, 268, 61–69.
Stein, T. (2005). Bacillus subtilis antibiotics: structures, syntheses and specific functions. Molecular Microbiology,
56, 845–857.
Surette, M.G., Miller, M.B., and Bassler, B.L. (1999).
Quorum sensing in Escherichia coli, Salmonella typhimurium, and Vibrio harveyi: a new family of genes responsible for autoinducer production. Proceedings of the
National Academy of Sciences of the USA, 96, 1639–1644.
Taga, M.E., Semmelhack, J.L., and Bassler, B.L. (2001).
The LuxS-dependent autoinducer Al-2 controls the
expression of an ABC transporter that functions
in Al-2 uptake in Salmonella typhimurium. Molecular
Microbiology, 42, 777–793.
Tateda, K., Ishii, Y., Horikawa, M. et al. (2003). The
Pseudomonas aeruginosa autoinducer N-3-oxododecanoyl
homoserine lactone accelerates apoptosis in macrophages and neutrophils. Infection and Immunity, 71,
5785–5793.
Telford, G., Wheeler, D., Williams, P. et al. (1998). The
Pseudomonas aeruginosa quorum-sensing signal molecule N-(3-oxododecanoyl)-1-homoserine lactone has
immunomodulatory activity. Infection and Immunity,
66, 36–42.
Velicer, G.J. and Yu, Y.T.N. (2003). Evolution of novel
cooperative swarming in the bacterium Myxococcus
xanthus. Nature, 425, 75–78.
Venturi, V. (2006). Regulation of quorum sensing
in Pseudomonas. FEMS Microbiology Reviews, 30,
274–291.
Visick, K.L., Foster, J. Doino, J., McFall-Ngai, M., and Ruby,
E.G. (2000). Vibrio fischeri lux genes play an important
role in colonization and development of the host light
organ. Journal of Bacteriology, 182, 4578–4586.
Wagner, V.E., Bushnell, D., Passador, L., Brooks, A.I.,
and Iglewski, B.H. (2003). Microarray analysis of
Pseudomonas aeruginosa quorum-sensing regulons:
effects of growth phase and environment. Journal of
Bacteriology, 185, 2080–2095.
West, S.A. and Buckling, A. (2003). Cooperation, virulence
and siderophore production in bacterial parasites.
Proceedings of the Royal Society Series B: Biological
Sciences, 270, 37–44.
DettoreBook.indb 31
31
West, S.A., Kiers, E.T., Simms, E.L., and Denison, R.F.
(2002a). Sanctions and mutualism stability: why do
rhizobia fix nitrogen? Proceedings of the Royal Society
Series B: Biological Sciences, 269, 685–694.
West, S.A., Pen, I., and Griffin, A.S. (2002b). Conflict and
cooperation—cooperation and competition between
relatives. Science, 296, 72–75.
West, S.A., Griffin, A.S., Gardner, A., and Diggle, S.P.
(2006). Social evolution theory for microorganisms.
Nature Reviews Microbiology, 4, 597–607.
West, S.A., Diggle, S.P., Buckling, A., Gardner, A., and
Griffin, A.S. (2007a). The social lives of microbes. Annual
Review of Ecology, Evolution and Systematics, 38, 53–77.
West, S.A., Griffin, A.S., and Gardner, A. (2007b). Social
semantics: altruism, cooperation, mutualism, strong
reciprocity and group selection. Journal of Evolutionary
Biology, 20, 415–432.
Williams, P., Winzer, K., Chan, W.C., and Camara, M.
(2007). Look who’s talking: communication and quorum sensing in the bacterial world. Philosophical
Transactions of the Royal Society B: Biological Sciences,
362, 1119–1134.
Winans, S.C. (2002). Bacterial Esperanto. Nature Structural
Biology, 9, 83–84.
Winson, M.K., Camara, M., Latifi, A. et al. (1995). Multiple
N-acyl-L-homoserine lactone signal molecules regulate
production of virulence determinants and secondary
metabolites in Pseudomonas aeruginosa. Proceedings of the
National Academy of Sciences of the USA, 92, 9427–9431.
Winzer, K., Hardie, K.R., and Williams, P. (2002). Bacterial
cell-to-cell communication: Sorry, can’t talk now—gone
to lunch! Current Opinion in Microbiology, 5, 216–222.
Winzer, K., Hardie, K.R., and Williams, P. (2003). LuxS
and autoinducer-2: their contribution to quorum sensing and metabolism in bacteria. Advances in Applied
Microbiology, 53, 291–296.
Wisniewski-Dye, F. and Downie, J.A. (2002). Quorumsensing in Rhizobium. Antonie Van Leeuwenhoek
International Journal of General and Molecular Microbiology,
81, 397–407.
Xavier, J.B. and Foster, K.R. (2007). Cooperation and conflict in microbial biofilms. Proceedings of the National
Academy of Sciences of the USA, 104, 876–881.
Yates, E.A., Philipp, B., Buckley, C. et al. (2002).
N-acylhomoserine lactones undergo lactonolysis in a
pH-, temperature-, and acyl chain length-dependent
manner during growth of Yersinia pseudotuberculosis
and Pseudomonas aeruginosa. Infection and Immunity, 70,
5635–5646.
Zhang, L.H., Murphy, P.J., Kerr, A., and Tate, M.E. (1993).
Agrobacterium conjugation and gene-regulation by
N-acyl-L-homoserine lactones. Nature, 362, 446–448.
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